How to make displacement working on imported objects?
Three things for this to work

Proper UV maps (main reason)
- your imported objects have broken or even missing UV maps, so the displacement doesn't work
- to solve this, select all faces of the mesh and go UV > Smart UV Project  

Connect Displacement texture
- your node setup is very... unusual :)
- you need to connect your Displacement Texture directly into the Displacement node

Apply scale
- some of your objects have very small scale (0.01) which also affects final displacement
- apply the scale through Object > Apply > Scale  so it 1.

Apply scale to parented objects
- some object's are parented - you need to clear the parent before applying scale
- select parented object and go RMB > Clear and Keep Transformation



How to create a simple UV map.
This is just a quick and dirty way. Learn to unwrap objects manually



How to connect Displacement texture



How to clear parent and apply scale
Do this for all imported objects, just to be sure